The separation of multicomponent gas mixtures by pressure swing adsorption (PSA) is a well-established and widely-practiced technology. A large body of prior art in this field pertains to the recovery of a single component at high purity from a mixture of that component with impurities present at lower concentrations, or the recovery of one product at high purity and a second product at low purity from a gas mixture. One example of the former is the recovery of high-purity hydrogen from petroleum refinery gas streams; an example of the latter is the separation of air into a medium- to high-purity nitrogen stream and a low-purity, oxygen-rich stream. In these types of separations, parallel adsorbent beds are operated sequentially as parallel sets of single-stage adsorbers.
There is a smaller body of prior art in which at least two products, in some cases both at high purity, are recovered from a gas mixture in a series of staged or semi-staged adsorbent beds, each stage of which is selective for a different component or group of components in the mixture. U.S. Pat. No. 3,252,268 discloses a PSA system comprising three adsorption stages in which a product is recovered in part at moderately high purity and in part at ultra-high purity. In this system, the first stage adsorbs most of the major contaminants in the feed stream, the second stage selectively adsorbs at least one impurity component in the effluent from the first stage, and the third stage selectively adsorbs at least one impurity component in the effluent from the second stage. An ultra-high purity product is recovered as the effluent from the third stage adsorber, and a moderately pure product is recovered as effluent from the second stage adsorber and as depressurization gas from the third stage adsorber. As an example, ultra-pure hydrogen at 99.995% purity and moderately-pure hydrogen at 99% purity are recovered from refinery off gases by the disclosed process.
U.S. Pat. No. 4,171,207 discloses a PSA system comprising two groups of adsorbent beds for the recovery of two high-purity products from a gas mixture containing a primary key component, a secondary key component which is more strongly adsorbed than the primary key component, and one or more tertiary components at low concentrations which are more strongly adsorbed than the primary or secondary key components. The gas mixture is passed through a first adsorbent bed in which the tertiary components are selectively adsorbed, and then through a second adsorbent bed in which the secondary key component is selectively adsorbed; a product stream of high-purity primary key component is withdrawn as an effluent stream from the second adsorbent bed. Flow through the two beds is then discontinued, and the second bed is rinsed at feed pressure with a stream of high-purity secondary key component. This rinse step displaces other components present in the bed and saturates the bed with secondary key component. High-purity secondary key component then is recovered by desorption from the second adsorbent bed. An example of this process is the adsorptive separation of a mixture of hydrogen, methane, and C.sub.2.sup.+ hydrocarbons to recover high-purity hydrogen as the primary key component and high-purity methane as the secondary key component.
U.S. Patent No. Re. 31,014, a reissue of U.S. Pat. No. 4,171,206, discloses a PSA system comprising two sets of adsorbent beds for the recovery of two high-purity products from a gas mixture containing a primary key component, a secondary key component which is more strongly adsorbed than the primary key component, and one or more tertiary components at low concentration which are less strongly adsorbed than the secondary key component and more strongly adsorbed than the primary key component. The gas mixture is passed through a first adsorbent bed in which the secondary key component is selectively adsorbed, and then through a second adsorbent bed in which the tertiary components are selectively adsorbed; a product stream of high-purity primary key component is withdrawn as an effluent stream from the second adsorbent bed. Flow through the two beds is then discontinued, and the first bed is rinsed at feed pressure with a stream of high-purity secondary key component. This rinse step displaces other components present in the bed and saturates the bed with secondary key component. High-purity secondary key component is then recovered by desorption from the first adsorbent bed. An example of this process is the adsorptive separation of a mixture of hydrogen, carbon dioxide, carbon monoxide, methane, and nitrogen to recover high-purity hydrogen as the primary key component and high-purity carbon dioxide as the secondary key component.
Japanese Patent Application No. 58-205,592 discloses a three-stage PSA process for the recovery of three individual products from a multicomponent gas mixture comprising hydrogen, carbon monoxide, carbon dioxide, and nitrogen. Such a gas mixture is recovered, for example, from a steel converter furnace. The process comprises three separate multi-bed PSA systems arranged in series; the feed gas is passed through the first stage PSA in which CO.sub.2 is preferentially adsorbed, through the second stage PSA in which CO is preferentially adsorbed, and then through the third stage PSA in which N.sub.2 and residual CO are preferentially adsorbed. Hydrogen product is recovered as third stage effluent. CO.sub.2 is recovered by desorption from the first stage and CO is recovered, after a rinse step, by desorption from the second stage. An example is disclosed in which CO is recovered at 99.4% purity and H.sub.2 at 99.9% purity. CO.sub.2 purity is not disclosed, but based upon the process steps used in the first PSA, would be at low purity compared with recovered CO and H.sub.2. The three PSA systems operate independently, except for the use of the third stage waste stream comprising N.sub.2 and CO as purge gas in the first stage. No intersystem pressure equalization steps are used in the process.
U.S. Pat. No. 4,539,020 discloses a two-stage PSA process for recovering carbon monoxide from a feed gas comprising carbon monoxide, carbon dioxide, and components less adsorbable than carbon monoxide. Feed gas is passed to a first PSA system in which CO.sub.2 is selectively adsorbed and recovered by the steps of adsorption, depressurization, evacuation, purging with waste gas from the second stage PSA system, and repressurization with first stage product gas. Effluent from the first stage PSA is introduced into the second stage PSA in which carbon monoxide is selectively adsorbed and recovered by the steps of adsorption, partial depressurization, pressure equalization, purge with product CO, evacuation yielding product CO, pressure equalization, and final repressurization to the adsorption pressure with purge effluent from another adsorbent bed in the second stage PSA system. A CO.sub.2 -rich waste gas is produced by the first stage PSA; the CO product from the second stage PSA contains less than 0.5% CO.sub.2 and less than 1% N.sub.2.
U.S. Pat. No. 4,790,858 discloses an improvement to previously cited U.S. Patent Re. 31,014 wherein additional primary key component is recovered by passing the waste stream from the second set of adsorbent beds to a third set of adsorbent beds in which additional secondary key tertiary components are selectively adsorbed. The additional recovered primary key component from the third adsorbent bed is used in part for purging the second adsorbent bed, and for purging and repressurizing another third adsorbent bed.
A distinguishing feature of the earlier cited U.S. Pat. No. 4,171,207 and U.S. Patent No. Re. 31,014 is the high-pressure rinse step in which high-purity secondary key component is utilized to rinse at the adsorption pressure the adsorbent bed in which the secondary key component has been selectively adsorbed. This step is important in the recovery of secondary key component at high purity.